An electrochemical cell is a device capable of providing electrical energy from an electrochemical reaction, typically between two or more reactants. Generally, an electrochemical cell includes two electrodes, called an anode and a cathode, and an electrolyte disposed between the electrodes. In order to prevent direct reaction of the active material of the anode and the active material of the cathode, the electrodes are electrically isolated from each other by a separator.
In one type of electrochemical cell, sometimes called a hydrogen fuel cell, the anode reactant is hydrogen gas, and the cathode reactant is oxygen (e.g., from air). At the anode, oxidation of hydrogen produces protons and electrons. The protons flow from the anode, through the electrolyte, and to the cathode. The electrons flow from the anode to the cathode through an external electrical conductor, which can provide electrical energy. At the cathode, the protons and the electrons react with oxygen to form water.
In another type of electrochemical cell, called a metal-air cell, oxygen is reduced at the cathode, and a metal (e.g., zinc) is oxidized at the anode. Electrons flow from the anode to the cathode through an external electrical conductor, which can provide electrical energy. Oxygen can be supplied to the cathode from the atmospheric air external to the cell through one or more air hole(s) in the cell housing. An electrolytic solution (e.g., an alkaline electrolyte, such as a potassium hydroxide solution) in contact with the electrodes contains ions that flow through the separator between the electrodes to maintain charge balance throughout the cell during discharge.
Metal-air cells can experience carbonation, in which the alkaline electrolyte in the cathode absorbs carbon dioxide, resulting in the precipitation of carbonate salts (such as potassium carbonate or sodium carbonate). These salts can have a detrimental effect on the cell by, for example, blocking cathode pores or air access holes on the cathode side of the cell envelope. A result can be that the cathode has less access to the oxygen it needs to function.
Furthermore, a metal-air cell can experience water exchange with its environment, as a result of the difference in the relative humidity of the environment and the equilibrium vapor pressure of the cell electrolyte. When the ambient air is drier (i.e., has a lower partial pressure of water vapor) than the electrolyte, the cell can lose water to the environment and dry out. On the other hand, when the ambient air is wetter (i.e., has a higher partial pressure of water vapor) than the electrolyte, the cell can gain water, such that the cathode ultimately floods with electrolyte solution. In either case, a consequence is that the cell can lose its ability to support heavy currents. Additionally, when the cathode is flooded with electrolyte solution, the electrolyte solution can eventually leak out of the air access holes.